1. Field of the Invention
This invention relates to toners for electrostatic image development, particularly to a toner which can provide clear developed images over a long use cycle.
2. Description of the Prior Art
Generally, in electrophotography a colored resin powder, i.e., toner, is spread on a photosensitive plate or paper containing an electrostatically charged image. The toner particles image-wise adhere to the surface of the photosensitive plate or paper because of the electrostatically charged image. In this manner a visible image is developed on the plate or paper. The developer image may be transferred from the photosensitive plate on to a copy paper. The toner particles are thermally fused to the surface of the photosensitive paper or copy paper, thereby fixing the visible image on the paper.
The above technique is called the dry development method. In this method the toner is mixed with a carrier in order to electrically charge the toner to the degree necessary and spread the toner particles on the photosensitive plate or paper in uniform concentration. There are two dry development methods. One is called the cascade method in which a mixture of toner and a carrier such as glass particles, i.e., developing agent, is applied onto a photosensitive plate or paper. The other is called the magnetic brushing method in which a magnetic brush is used. The magnetic brush is comprised of a rod magnet to which the mixture of toner and carrier such as ferro magnetic particles, i.e., one of components in the developing agent, is attracted. The brush with the developing agent is so moved as to sweep the surface of a photosensitive plate or paper. During this sweeping process only the toner particles fall from the brush and stick to the surface of the photosensitive plate or paper.
In either dry development method, however, the surface of each carrier particle can never be free of toner particles since in the developing agent the toner particles adhere to the carrier particles. If not free of toner particles, the carrier fails to be charged electrically to the degree necessary, thus degrading the quality of the developed image. Further, in the magnetic brushing method toner particles will also adhere to those portions of the photosensitive plate or paper where no image has been electrostatically formed. If not free of extraneous toner particles, the photosensitive plate or paper degrade greatly the quality of the developed electrostatic image, particularly when it is repeatedly used and subjected to electrical charging, exposure, development and transfer many times.
Moreover, the toner contains a thermoplastic resin binder which melts or softens at a relatively low temperature. While being used or stored, the toner may therefore easily fuse together to form a massive block of material form agglomerates or the like depending upon the ambient temperature. If this happens, the toner loses its function as a powder toner.
A one component toner, that is one which does not contain carrier material, comprises in a single particle both the thermoplastic resin and magnetite powder. The attractive force of the magnetic brush for such powders is, of course, a function of the mass of magnetite powder in the toner particles. Thus such toner particles must contain a significant quantity of magnetite powder.
Accordingly, a need continues to exist for a toner having the proper balance of necessary properties.